If you find an issue with the cap, not latest cross threaded etc, then fill tank to high level with distilled water. You can lift the bail to purge on the drive to the shop. In general the older caps get weak and will leak when SD due to the residual thermal expansion. I'd fill and drive 7-8 miles no problem, although flatbedding would certainly be safer.
The following excerpt from the owners manual compliments of 78F350 to explain the timer.
